Optics Topics

2.9979e8 ms-1
Velocity of light in vacuo.
Rays of light which do not coincide with the optical axis of a lens system.
Abbe Number
The ratio of refractivity to dispersion in an optical medium.
Abbe Refractometer
An instrument for measuring directly the refractive index of liquids, minerals and gemstones.
A defect in a mirror or lens causing light rays from a single point to fail to focus at a single point in space.
Abney Mounting
A form of mounting for a concave diffraction grating, in which the eye-piece is fixed at the centre of curvature of the grating and the slit can move around the circumference of the Rowland circle, to bring different orders of spectrum into view.
To transform radiant energy into a different form, with a resultant rise in temperature.
Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength.
Absorption Coefficient
The fraction of sound energy that is absorbed at any surface.
Absorptive Index
The imaginary part of the complex index of refraction of a medium.
Accessible Emission Level
Magnitude of accessible laser radiation of a specific wavelength or emission duration at a particular point.
Achromatic Lens
Lens for which all light colours have the same focal length.
Pertaining to electromagnetic radiation capable of initiating photochemical reactions.
Actinic Ray
A ray of light of short wavelengths, as ultraviolet or violet, that produces photochemical effects.
Adaptive Optics
A technique that uses deformable mirrors on a telescope to correct the blurring caused by turbulence in the atmosphere.
Additive Primaries
Red, green, and blue are the primary colours of light from which all other colours can be made.
After Image
An image seen after the eye′s retina has been exposed for a time to an intense or stationary light source.
Airy Disc
A pattern of illumination caused by diffraction at the edge of a circular aperture.
That part of an optical measuring instrument comprising the optical system, indicator, vernier, etc.
Distorted, as in an optical system with different magnification levels or with focal lengths perpendicular to the optical axis.
Angle of Deviation
The angle through which a ray is bent by refraction.
Angle of Incidence
Angle between direction of motion of waves and a line perpendicular to the surface the waves are striking.
Angle of Reflection
Angle between direction of motion of waves and a line perpendicular to surface the waves are reflected from.
Angle of Refraction
The angle at which a refracted ray of energy leaves the interface at which the refraction occurred.
It is an old unit used for measuring the wavelength of electromagnetic radiation including visible light and X-rays.
Anti Reflection Coating
A thin layer of material that, when applied to a lens, increases its transmittance and reduces its reflectance.
An opening through which radiation can pass.
A unit of surface luminance used when defining diffusing surfaces equal to 1 lumenm-2
Arago Spot
A bright spot that appears in the shadow of a uniform disc being backlit by monochromatic light emanating from a point source.
ASA Number
A measure of the speed of the photographic emulsion.
Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one or more surfaces are aspherical.
An aberration, or defect, in a mirror or lens that causes the image of a point to spread out into a line.
Atmospheric Windows
The spectral bands in which the atmosphere least affects the transmission of radiant energy.
A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light.
Autokinetic Illusion
The illusion of a fixed object or light moving when gazed at steadily.
Axial Flow Laser
An axial flow of gas is maintained through the tube to replace those gas molecules depleted by the electrical discharge used to excite the gas molecules to the lasing.
Axicon Lens
A conical lens which, when followed by a conventional lens, can focus laser light to a ring shape.
Back Focal Length
The distance between the last surface of a lens to its back focal plane.
Bandpass Filter
An electronic network which transfers through it a specific band of signal frequencies.
The bandwidth of a filter is the separation between the lower and upper frequencies at which the amplitude of a sinusoidal signal is attenuated by a factor of 2.
Beam Splitter
An optical device using controlled reflection to produce two beams from a single incident beam.
The ratio of the curvatures of a lens′s two refracting surfaces.
Bi-Concave Lens
Having two outer surfaces that curve inward.
Bi-Convex Lens
Having two outer surfaces that curve outward.
The separation of a beam of light into two beams as it passes through a doubly refracting material or object.
Blur Circle
The image of a point-source object formed by an optical system on its focal surface.
Bouguer Law
A relationship describing the rate of decrease of flux density of a plane-parallel beam of monochromatic radiation as it penetrates a medium which both scatters and absorbs at that wavelength.
The visual sensation of the luminous intensity of a light source.
Broadband Coating
A multi-layer antireflection coating with reduced reflection over a broad spectral band.
An instrument for taking photographs consisting of a lightproof box, shutter, adjustable aperture and a lens through which an image is focussed on a light sensitive film or sensor.
Camera Lens
An arrangement of one or more pieces of optical glass designed to collect and focus rays of light to form a sharp image on the film.
Camera Obscura
A room with a small hole in one wall used by artists to produce images.
The SI unit of luminous intensity.
Carbon Dioxide Laser
A widely used laser in which the primary lasing medium is carbon dioxide gas.
Cassegrain Telescope
Two-mirror reflecting telescope.
An optical system containing both reflective and refractive elements.
Abbreviation of Charge Coupled Device.
Abbreviation of Closed Circuit Television Lens.
An abbreviation of candela, the SI unit of luminous intensity.
Centre Wavelength
For filters and coatings, the average of the wavelength values at the half-power points of the transmission band.
Charge Coupled Device
A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor, surface storage and information transfer technologies.
Chromatic Aberration
A defect in lenses that causes different colors (wavelengths) of light to have different focal lengths.
The colour portion of the video signal that includes hue and saturation information but not brightness.
Closed Circuit Television Lens
A family of compound lenses which offer exceptionally high resolution, short focal length and color imaging at fast lens apertures, such as required in the television industry.
CCD camera lens connector thread 1"-32UN2A with a standard focal distance of 17.526mm.
CO2 Laser
Abbreviation of Carbon Dioxide Laser.
A property of two or more sources of waves that have the same wavelength and maintain constant phase differences.
Coherent Anti-Stokes Raman Scattering
A phenomenon observed in the scattering of light as it passes through a transparent medium. The light undergoes a change in frequency and a random alteration in phase due to a change in rotational or vibrational energy of the scattering molecules.
Collimated Light
Light rays that are parallel.
The alignment of the direction of the photons, so the beam of radiation can be directed at a well-defined part of a target material.
A mechanical device installed along the trajectory of a beam to reduce the size of the beam.
A quality of light, depending on its wavelength. Spectral colour of an emission of light is its place in the rainbow spectrum.
Complementary Colours
For lights, two colours that combine to form white.
Concave Lens
Lens thinner in centre than edges; a diverging lens.
Confocal Imaging
Technique that uses a source point and a confocal point to image an object.
Converging Lens
Lens that causes light rays to converge; usually a convex lens.
Convex Lens
Lens that is thicker in center than at edges.
Convolution meaning ..intertwined, coiled…
Critical Angle
The minimum angle of incidence for which total internal reflection occurs.
CCD camera lens connector thread 1"-32UN2A with a standard focal distance of 12.5mm.
Departure from flatness of a surface. Defined as the reciprocal of the radius of curvature.
Depth of Field
The distance along the optical axis through which an object can be located and clearly defined when the lens is in focus.
Depth of Focus
The distance along the optical axis through which an image can be clearly focused.
A chiral molecule which rotates plane-polarized light to the right.
Optical isomers that are not mirror images of each other.
The spreading of waves passing through an opening or around a barrier.
Diffraction Grating
Grating device that splits light into a spectrum of the component wavelengths.
Diffraction Limited
Describes an optical system in which the quality of the image is determined only by the effects of diffraction and not by lens aberrations.
Diffuse Reflection
The reflection of rays from a rough surface. The reflected rays do not leave at fixed angles.
Digital Camera
An electronic camera that captures images using a solid-state image sensor, and then outputs a digital signal representing the images or records the images on a digital storage medium.
The spreading of light into a spectrum of colour. The variation in the speed of a periodic wave due to its wavelength or frequency.
Dots Per Inch
A unit used to measure the resolution, or sharpness of an image.
Abbreviation of Dots Per Inch.
Electromagnetically Induced Transparency
A technique that makes normally opaque substances transparent to light by applying another source of light, such as a laser beam, that interferes with the opaque substance′s absorption of light.
Emission Spectrum
The collection of discrete wavelengths emitted by atoms that have been excited by heating or by electric currents.
The ratio of the radiant energy emitted by a any source to that emitted by a blackbody at the same temperature.
Extreme Ultraviolet, a portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Extended Source
An extended source of radiation can be resolved into a geometrical image in contrast with a point source of radiation, which cannot be resolved into a geometrical image.
Extreme Ultraviolet
A portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Hyperopia. Images of distant objects are formed beyond the retina.
Fata Morgana
A complex mirage display that involves multiple images.
Fermat′s Principle
The path taken by a ray of light between any two points in a system is always the path that takes the least time.
Fibre Coating
Material immediately covering the optical fibre to preserve the integrity of the fibre.
Fibre Optics
The transmission of radiant energy through transparent fibres of glass, plastic or fused silica.
Field of View
The maximum visible space seen through an optical instrument or lens.
Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
First Order Optics
Optical characteristics limited to infinitesimally small pencils of light.
Impression of fluctuating luminance or colour.
Fluid Opacity
Related to the ability of a fluid to pass light.
Focal Length
The distance from a mirror or the centre of a lens to its focal point.
Focal Plane
A surface upon which the image of all points in the field of view of an optical instrument is created.
Focal Point
The location at which a mirror or a lens focuses rays parallel to the optic axis or from which such rays appear to diverge.
Point at which converging rays meet and at which a clearly defined image can be obtained.
Foot Candle
A measure of light intensity on a surface being illuminated.
The central portion of the retina where visual acuity, or the ability to distinguish small objects and details, is greatest. Only about half a millimetre in diameter, the fovea is the retina′s "rod-free zone" and is densely packed with cones.
Fraunhofer Diffraction
Fraunhofer diffraction describes the diffraction pattern of electro-magnetic radiation observed in the far field from a slit or series of slits (i.e. grating).
Fresnel Reflection
Reflection occurring at the planar junction of two materials having different indices of refraction.
The ratio of the focal length of the lens to diameter of the aperture.
Fused Quartz
The glassy isotropic form of quartz.
Fused Silica


The glassy isotropic form of quartz.

Gas Laser
A type of laser in which the laser action takes place in a gas medium.
Gaussian Optics
Optical characteristics limited to infinitesimally small pencils of light.
Geometric Optics
That branch of optics dealing with the tracing of ray paths through optical systems.
Graded Index Fibre
An optical fibre whose core refractive index decreases from its centre out to its edge.
Group Index
The ratio of the vacuum velocity of light to the group velocity in the medium.
Helium-Neon Laser
A laser in which the active medium is a mixture of helium and neon.
HeNe Laser
Abbreviation of Helium-Neon Laser.
A three-dimensional record of visual information.
The photographic process for producing three-dimensional images.
Farsightedness. Images of distant objects are formed beyond the retina.
This is defined as the luminous flux arriving at a surface that is perpendicular to it per unit area.
Image Circle
The circular image field over which image quality is acceptable, can be defined in terms of its angular subtense.
Image Inversion
Change in the orientation of an image in one meridian.
Image Plane
The plane perpendicular to the optical axis at the image point.
Incident Wave
The wave that strikes the surface of a medium.
Index Matching Gel
Material whose index of refraction is almost equal to that of the fibre core and is used to minimize Fresnel reflections.
Index of Absorption
The imaginary part of the complex index of refraction of a medium. Also known as Absorptive Index.
Index of Refraction
This is the ratio of the velocity of light in free space to that in the material.
Inferior Mirage
Mirage in which the inverted image is below the normal one.
Infrared Amplification by Stimulated Emission of Radiation
A device which amplifies in the infrared band.
Interference Filter
A filter which controls the spectral composition of transmitted energy by interference.
The design and use of optical interferometers.
Referring to something being upside down.
Ion Laser
A type of laser employing a very high discharge current, passing down a small bore to ionize a noble gas such as argon or krypton.
Abbreviation of Infrared Amplification by Stimulated Emission of Radiation.
The pigmented fibrovascular tissue known as a stroma. It is the most forward portion of the eye and the only one seen on superficial inspection.
Irradiation Illusion
An illusion of visual perception in which a light area of the visual field looks larger than an otherwise identical dark area.
K.D.P. (Potassium D Phosphate) is a crystal often used in non-linear optics within Pockles Cells, Kerr Cells to vary the polarisation and phase of light. Also used as a frequency doubling crystal in lasers.
Kerr Cell
A Kerr Cell is most commonly used as a Q-Switch device in pulsed lasers
Kerr Effect
The ability of certain substances to differently refract light waves whose vibrations are in different directions when the substance is placed in an electric field.
Lamberts Laws
The illuminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source.
An acronym for light amplification by stimulated emission of radiation.
Laser Diode
A laser which uses a forward biased semiconductor junction as the active medium.
A lens is an optical component whos surface curvature and refractive index are designed such that the direction of light incident upon it is controlled.
Lens Mount
A mechanical and often also electrical interface between a photographic camera body and a lens.
Lens Thread M39
Also known as the Leica L-Mount. This is 39mm x 26tpi.
Lens Thread M42
This is an metric 42x1.0mm thread used for single reflex cameras before the bayonet couplings became commonplace.
Lens Thread T2
42mm x 0.75mm lens adaptor thread used by Tamron.
A chiral molecule which rotates plane-polarized light to the left.
Electromagnetic radiation that is visible to the unaided human eye.
Light Beam
A collection of rays that may be parallel, convergent, or divergent.
Light Ray
A line that represents the path of light in a given direction.
Light Reflectance
The measure of light reflectance is that fraction of the specified incident light is reflected by the surface.
Light Scattering
When light passes through a medium that is not perfectly homogeneous, the irregularities in the medium scatter some of the light in all directions.
Limit of Resolution
The limit to the performance of a lens imposed by the diffraction pattern resulting from the finite aperture of the optical system.
Linke Turbidity Factor
A measure of atmospheric turbidity, equal to the ratio of total optical depth to the Rayleigh optical depth.
An abbreviation of lumen, the derived SI unit of luminous flux.
lm h
Abbreviation of lumen hour.
Lossy Medium
A medium which absorbs or scatters radiation passing through it.
The derived SI unit of luminous flux.
lumen hour
A unit of quantity of light, equal to one lumen of light flux continued for one hour.
An abbreviation of lumen m-2, the metric unit of illuminance.
An abbreviation of lux that itself is an abbreviation of lumen m-2, the metric unit of illuminance.
Curvature of a fibre which causes loss of light.
The enlargement of an object by an optical instrument.
Malus′ Law
Malus′ law defines the transmitted intensity through 2 polarisers for a given angle between the transmission directions of said polarisers.
Microscopic bends and bumps in the fibre which cause loss of light by scattering.
An optical effect that produces an image that looks like it has been reflected from the surface of a body of water.
Mode Coupling
Energy exchange between modes of light propagating in an optical fibre.
Moire Fringes
A pattern formed by the interference between two regular sets of divisions.
Radiation that has a single wavelength.
Multi Mode Fibre
Optical fibre supporting more than one propagating mode.
Nearsightedness. Images of distant objects are formed in front of the retina.
Nd YAG Laser
Abbreviation of Neodymium Yttrium Aluminum Garnet Laser.
Myopia. Images of distant objects are formed in front of the retina.
Neodymium Yttrium Aluminum Garnet Laser
A synthetic crystal used as a laser medium to produce 1064 nm light.
Neutral Density
An optical coating that appears grey to the eye and has a flat absorption curve throughout the visible spectrum.
Newtonian Telescope
A reflecting telescope in which a small plane mirror reflects the convergent beam from the objective to an eyepiece at one side of the telescope.
Nonluminous Bodies
Objects that either reflect or diffuse light that falls upon them.
Non-Specular Surface
A surface which is diffuse, meaning it reflects light in equal amounts in all directions.
The optical element which receives light from the object and forms the first or primary image in telescopes, microscopes and other optical systems.
Pertaining to or in relation with the eye.
Referring to movements of the eyes.
The condition of being non-transparent.
Substances that do not transmit (pass) any light rays.
Optic Axis
A line passing through the center of a curved mirror and the center of the sphere from which the mirror is made.
Optical Activity
A substance that is capable of rotating plane-polarized light.
Optical Alignment
A secondary alignment method for determining on-line and off-line changes in alignment conditions.
Optical Axis
A line passing through the centres of curvature of a lens or series of lenses in an optical system.
Optical Cavity
Space between the laser mirrors where lasing action occurs.
Optical Coupler
A coupler composed of an LED and a photodiode and contained in a lightconducting medium.
Optical Filter
A device used to reject certain frequencies of light.
Optical Flat
A piece of glass with one or both surfaces polished flat.
Optical Illusion
A drawing or object that appears to have an effect that it does not really have, such as when a flat painting seems to have three-dimensional depth.
Optical Interference
The interaction of two light waves on the total intensity of light.
Optical Pumping
The excitation of the lasing medium by the application of light rather than electrical discharge.
Optical Pyrometer
An instrument for measuring the temperature of heated material.
Optical Transmission
To admit the passage of light through a medium; light not reflected back to its source is transmitted through the medium.
Any device that functions as an electrical-to-optical or optical-to-electrical transducer.
Parabolic Reflector
An antenna reflector in the shape of a parabola.
Change in the apparent position of objects when viewed from two widely separated positions.
Paraxial Optics
Optical characteristics limited to infinitesimally small pencils of light.
The transition region between the darkest shadow and full brightness.
Perceived Colour
Also known as visual colour is the quality of light emission as conveyed by the human eye, combining the impressions of 3 types of light-sensitive cells which the eye contains.
Phase identifies the position at any instant which a periodic wave occupies in its cycle. A portion of a material system whose properties and composition are homogeneous and which is physically distinct from other parts of the system.
The study of light.
Luminescence caused by absorption of optical radiation.
An instrument which measures luminous intensity.
Massless packet of energy, which behaves like both a wave and a particle.
Visual representation of something.
A small, sharp-edge hole without a lens which can function as an aperture or eye lens.
Single picture element of a detection device.
Plane Wave
Wave in which the wavefronts are everywhere parallel planes normal to the direction of propagation.
Pockels Cell
A Pockels Cell is a device which contains a photo refractive crystal. When a voltage is applied to this crystal it can change the polarisation or phase of the light beam.
Point Source
Ideally, a source with infinitesimal dimensions.
Poisson Spot
Poisson originally predicted the existence of the arago spot, and used the prediction to demonstrate how the wave theory of light must be in error to produce such a counterintuitive result.
A property of a transverse wave when its vibrations are all in a single plane.
Power of a Lens
The power of a lens is defined as the reciprocal of its focal length, in metres.
A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted.
Waves traveling through a medium.
Propagation Anomaly
A change in propagation characteristics due to a discontinuity or abnormality in the medium of propagation.
Used to build up a high population inversion within pulsed lasers.
Q-Switched Laser
A laser which stores energy in the laser media to produce extremely short, extremely high intensity bursts of energy.
Radiant Exposure
The total energy per unit area incident upon a given surface.
Radiosity Equation
Used in the construction of realistically lit scenes in computer graphics.
Ray Tracing
The calculation of the propagation, reflection and refraction of rays from source to receiver. This technique is used extensively within the optics and acoustics industry.
Rayleigh Criterion
A criteria that gives an approximation for how finely a set of optics may be able to distinguish.
Real Image
An image formed by the convergence of light.
Reciprocal Megakelvin
A unit used in colorimetry and photography to measure the wavelength of light, especially for selecting filters to adjust the colour temperature.
Reflecting Telescope
A type of telescope using a mirror as the objective.
Refracting Telescope
A type of telescope using a lens as the objective.
Abbreviation of Red, Green, Blue.
An optical technique that detects density gradients occurring in a fluid flow.
Darkness in a region, caused by an obstruction between the source of light and the region.
A picture or image in which steep density gradients in the flow about a body are made visible.
A device for controlling the amount of time a light-sensitive medium is exposed to light.
Single Mode Fibre
Optical fibre supporting only one mode of light propagation.
Snells Law
Snells law defines the angle of refraction of a light beam when it passes from one medium to another.
Solar Air Mass
The optical air mass penetrated by light from the sun for any given position of the sun in the sky.
Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.
An instrument that allows viewing of rotating or reciprocating objects by producing the optical effect of a slowing down or stopping motion.
Surface Plasmon
A collective motion of electrons in the surface of a metal conductor, excited by the impact of light of appropriate wavelength at a particular angle.
Telecentric Lens
A multi-component system whose aperture stop is located at the front focus so that the chief rays are parallel to the optical axis in image space.
Instrument used to focus electromagnetic radiation into an image.
Thermal Imaging Camera
A camera that detects infrared radiation and renders it as visible light thus allowing you to see the temperature of objects in the environment.
The detection and display of infrared radiation.
Those substances, such as frosted glass, through which some light rays can pass but through which objects cannot be seen clearly.
Those substances, such as glass, that pass almost all of the light waves falling upon them.
Transverse Electromagnetic Mode
The radial distribution of intensity across a beam as it exits the optical cavity.
Transverse Ray Error
Errors, or departures from ideal, measured in a direction perpendicular to the optical axis.
Tripod Mounts
The thread on tripod mounts is 1/4"-20 UNC, which is the smaller more common size. For larger cameras the 3/8"-16 UNC thread is used.
Truncation Ratio
The dimensionless ratio of the Gaussian beam diameter at the 1/e2 intensity point to the limiting aperture of the lens.
Tunable Dye Laser
A laser whose active medium is a liquid dye, pumped by another laser or flashlamps, to produce various colours of light.
Tunable Laser
A laser system that can be tuned to emit laser light over a continuous range of wavelengths or frequencies.
Ultraviolet Light
Electromagnetic radiation with wavelength longer than that of x-rays but shorter than that of visible light.
Abbreviation of ultraviolet.
The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures of elements within an optical system.
Vitreous Silica
The glassy isotropic form of quartz.
Wave Particle Duality
The observation that electrons; photons; and other very small entities behave like particles in some experiments and like waves in others.
Wavefront Deformation
Departure of a wavefront from ideal (usually spherical or planar) caused by surface errors or design limitations.
Wavelength Dispersion
Dispersion cause by the dependence of the phase and group velocities on wavelength due to geometric properties of the waveguide.
A piece of glass with plane parallel sides which admits light into or through an optical system and excludes dirt and moisture.
A widely used solid-state crystal which is composed of yttrium and aluminium oxides which is doped with a small amount of the rare-earth neodymium.
Young′s Double-Slit Experiment
A famous experiment which shows the wave nature of light.
Zenith Telescope
Instrument for observing stars near the zenith.
Zone System
A photographic system of exposure estimation.

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